The present invention relates to the field of high-speed data input schemes for integrated circuits, and in particular to techniques and circuitry for accurately sampling high frequency input data.
In integrated circuits such a microprocessors, memories, ASICs, and programmable logic devices (PLDs), it is desirable to input data at higher speeds. This allows higher performance integrated circuits, which in turn facilitates higher speed networks, such as the internet. One high speed interface scheme, among others, is known as low voltage differential signal (LVDS). LVDS is being used or will be used with many types of integrated circuits including programmable logic integrated circuits. The LVDS interface is especially useful for interfacing with fiber optic systems. Using LVDS, the data is input serially to the integrated circuit at a rate that is typically higher than the rate at which data is input at a regular data input. For example, a typical data input to the integrated circuit runs at about 80 megahertz while the LVDS data rate may be 8 times that rate.
In order to handle the LVDS data rate, the serial stream must be strobed at the appropriate times in order to correctly determine the data bits in stream. The user provides a input clock used to strobe the LVDS data stream. However, because of the high frequency of the LVDS data rate, the "window" where the LVDS data may be successful strobed is, for example, about 1.6 nanoseconds. Therefore, any skew between the input clock and LVDS input may lead to data errors since the wrong data will be strobed into the integrated circuit. This will lead to logic failures. Misalignment of the clock and input data may be caused by one or more of the following factors: PLL jitter, internal register set-up time and hold time, clock duty cycles, clock skew, process, and temperature variations, skew from the customer board design, and other considerations
Therefore, as can be appreciated, there is a need for interface circuitry and techniques for integrated circuits, especially programmable logic integrated circuits, to handle high frequency input data